Electroless plating



July 19, 1966 J. s. SALLO 3,261,711

ELEGTROLESS PLATING Filed Dec. 17, 1962 j 0 SUPPLY'".

W0 SUPPLTIJII PLATING RATE 0 INVENTOR. INCREASING H2PO2 CONCENTRATION (eRAMs/uTER) JfPflME 354M ATTORNEY United States Patent 3,261,711 ELECTRULESS PLATING Jerome S. Sallo, St. Louis Park, Minn, assignor to Honeyweli Inc, a corporation of Delaware Filed Dec. 17, 1962, Ser. No. 245,234 8 (Iiaims. (CI. 117-931) The present invention is directed to improvements in the electroless plating of nickel, cobalt or mixtures thereof.

Electroless plating of nickel, cobalt and nickel-cobalt alloys has been utilized extensively in recent years due to the unique capabilities of electroless plating not found in ordinary electrolytic plating. For example, electroless plating may be applied directly to nonconductive substrates and may also be used for plating the interior surfaces of parts without the necessity of using elaborate electroding of electrolytic plating.

In addition to the nickel, cobalt and nickel-cobalt electroless plating to which the present invention is specifically directed, the literature reports a large number of other electroless plating systems. Among these are arsenic, chromium, cadmium, copper, gold, iron and palladium. Various reducing agents have been suggested for use with these materials including the hypophosphite of the present invention as well as formate or formaldehyde in the case of cadmium and copper and hydrazine in the case of palladium.

Although electroless nickel and cobalt have been plated for many years from hypophosphite baths, the commercially used baths have not been wholly satisfactory for several reasons. Among these are relatively slow deposition rate (generally less than 1 mil per hour) and bath instability. It has been shown that plating rate is dependent upon the concentration of hypophosphite (with the exception of diffusion controlled baths) and that increasing the hypophosphite concentration will increase the rate of metal deposition. However, with increasing hypophosphite concentration, the tendency of the bath to decompose uncontrollably is also increased. By decomposition is meant that the bath uncontrollably creates minute particles of metal in the solution rather than only on the catalytic surface to be plated. Once formed, these particles in the solution, which are catalytic themselves, act as nucleation sites and bring about the rapid decompo sition of the bath and produce a finely divided black metal deposit throughout the bath.

Even in baths of nickel or cobalt having what is considered normal hypophosphite concentration, it has been found that these baths have relatively short life at the high temperatures (approximately 200 F.) at which these baths must necessarily be used. Numerous schemes have been proposed for eliminating this problem or reducing it. While partially successful, the schemes have not provided the advantages of the present invention.

The present invention provides both increased stability for electroless nickel, cobalt or nickel-cobalt plating baths generally and further permits use of far higher concentration of hypophosphite ion than has heretofore been possible. Some attendant advantages of this higher hypophosphite concentration will be discussed hereinbelow. Briefly, the preesnt invention resides in the maintaining of a saturated or near saturated concentration of oxygen within the bath during the plating period.

The present invention can best be understood from a study of the following specification and drawings wherem:

FIGURE 1 is a schematic illustration of a plating bath in accordance with the present invention;

FIGURE 2 is a schematic illustration of a second plating bath system in accordance with one modification of the present invention; and

Fatentedl July I9, 196% FIGURE 3 is a plot of relative plating rate versus hypophosphite concentration in a plating bath of the present invention wherein the oxygen concentration is near saturation.

I am aware that other investigators have proposed the bubbling of oxygen through copper electroless plating baths containing formaldehyde for increasing the stability of these baths. However, the problems encountered in the electroless plating of copper and the decomposition of these baths are quite dissimilar from those problems encountered in the use of nickel and cobalt .in electroless plating systems. Further, it has been shown in published work of the previous investigators that increasing the concentration of their reducing agent lowers plating rate rather than increases it as is the case with the present in vention. Of course, the copper deposit does not show magnetic properties as do certain of the deposits in accordance with the present invention.

While I do not intend to be bound to any theory as to the manner in which the present invention operates, the following appears to provide a reasonable explanation of the phenomena observed.

The present invention utilizes an at or near saturated oxygen condition to provide a type of bath stability against decomposition which will be referred to as class I.

Previous investigators have utilized certain additives in electroless nickel plating baths to provide a degree of bath stability against decomposition. This type of bath stability will be referred to as class II.

Understanding of the above two types of decompositionhomogeneous and heterogeneous-will prove helpful in distinguishing the present inventions method of stabilization from those previously known.

Homogeneous decomposition is the formation of free metal within the body of the bath in accordance with the following idealized reactions. It should be appreciated that the precise reactions may differ from the following. However, these reactions do show one explanation for the phenomena observed.

Heterogeneous decomposition is the formation of free metal at the surface of a catalytic metal in accordance with the following reaction:

III.

In the homogeneous reaction I, the H ion (or other reaction product) once formed is then capable of reacting with either water, to produce free hydrogen, or with the nickel ion to form nickel metal as in Equation II. I'have found that if a plating solution of nickel, cobalt or nickel-cobalt mixtures is maintained saturated with oxygen that the H" ion (or its equivalent product) will be oxidized in the bulk of the solution Without interference with the heterogeneous reaction of Equation III taking place upon the catalytic surface. In effect the oxygen successfully competes with the metal as an oxidizing agent for H. Once elemental metal has been formed in the bulk of the bath, the use of the saturated oxygen solution of the present invention will not serve to inhibit further reaction of the bath with the metal nucleation sites. The bath will only see the sites as additional catalytic surface.

Inhibitors of the class II type reactions have been studied by previous investigators. These previous investigators have proposed the use of materials such as thiourea in minute quantities. These agents act to decrease the rate at which the plating can occur on minute particles of metal which are already formed in the body of the plating bath. These agents apparently are absorbed on the surface of the already formed particle. Of course, these agents must be present in minute quantities only or they will inhibit the plating from forming in the parts being plated as well as preventing spontaneous decomposition on the minute nucleation sites. The inhibitor agents which are beneficial in stabilizing against class II decomposition are ineifective against class I decomposition. Accordingly, they do not permit the use of high concentrations of reducing agent and the attendant advantages which the present invention allows. However, when these agents are used in combination (as will be explained herein) with the saturated oxygen solution of my invention, certain advantages result.

It is an object of the present invention to provide an electroless plating bath for the plating of nickel, cobalt or cobalt-nickel alloys which is more stable than baths known heretofore.

It is a further object of the present invention to provide a plating bath of the above enumerated type which is capable of plating at higher speeds than prior art plating baths of these types.

It is a still further object of the present invention to provide a method of plating nickel-cobalt alloys which possess unique magnetic properties for use in recording and data storage devices.

Other and further objects of the present invention will become apparent from a study of the specification and claims.

As illustrative of the results achieved through the use of a saturated oxygen bath in the systems electroless nickel, electroless cobalt and electroless cobalt-nickel, the following tests and data are supplied.

An electroless nickel plating bath of the following composition was formulated.

Nickel sulfate hexahydrate gm./l 17 Sodium hypophosphite gm./l 24 Sodium acetate gm./l 41 pH 4.5 Acetic acid gm./l 30 This bath was heated to 205 F. It is important to realize at this time that the solubility of oxygen in water varies very strongly with increasing temperature. For example, at room temperature (25 C.) the solubility is equivalent to .004 gram per 100 grams water. At 205 F. (96.1 C.) the solubility is approximately .0004 gram per 100 grams of water. It is thus apparent that as the bath is heated from room temperature to an elevated temperature, a considerable quantity of the dissolved oxygen will escape. A bath prepared as above and maintained at 205 without the insertion of any catalytic substrate for plating decomposed uncontrollably after a period of about minutes. At this time, interior deterioration of the bath was noted with the forming of nucleation sites.

A second bath, prepared in the same manner and maintained at the same temperature with the exception that oxygen was bubbled (including the heat-up period) through the bath at a rate of 15 to bubbles per minute per liter, was stable indefinitely. (This is 3-4 ml./minute oxygen.)

The above results clearly show that the greatly enhanced stability can be obtained through the use of oxygen in an electroless nickel plating system. Similar tests were run on plating baths of cobalt electroless systems and of mixed cobalt and nickel electroless plating systems. As an example of the type of mixed system showing the same efifect, a bath of the following composition was made and tested as indicated above.

Cobalt chloride hexahydrate 33.3 gm./l. (.25 M). Nickel chloride hexahydrate gm./l. (.23 M).

Ammonium chloride 33.3 gm./l. (.62). Sodium hypophosphite 83.3 gm./l. (.78 M). Sodium citrate 100 gm./l. (.34). pH 7.5.

A bath prepared according to the above schedule would decompose before the operating temperature of 200 F. had been reached. However, if oxygen was passed through the bath during the heating stage, this bath shows indefinite stability.

While the above examples are to specific electroless nickel-cobalt or mixture baths and composition, it will be recognized that the concentrations of the various ingredients may be varied over wide limits. Likewise, various additives of the prior art may also be used in conjunction with the present invention. The prime advantage of my invention is the greatly enhanced stability of electroless nickel-cobalt, nickel and cobalt baths coupled with the permissible increase in hypophosphite ion concentration in these baths. It will generally be advantageous to increase the concentration of metal ion when the hypophosphite ion concentration is increased.

Two separate techniques have been utilized for maintaining the baths of the present invention essentially saturated with oxygen. In FIGURE 1 there is shown one such scheme where 10 generally indicates a container for the particular solution 11 being used for plating. 12 designates a heating means which may be a glass coil heating element. 13 is a manifold pipe having a number of fine openings for allowing the oxygen to escape therefrom and pass through the solution. 14 designates a filter for removing oil and other materials carried by the gas stream which may be deleterious to the bath. 15 is an oxygen source which may be ordinary air or bottled oxygen. In a system of this type the rate of passage of oxygen which will sufiice to provide effective stabilization of a system is quite broad and depends to an extent on the concentration of hypophosphite ion in the solution as will be described. In a bath of the simple electroless nickel plating type discussed above, a few milliliters per minute per liter of solution is sufficient to maintain stability. Larger amounts of oxygen are permissible but will tend to react with the hypophosphite and lower plating rate and further, will lower the overall efficiency of the bath insofar as grams of deposited metal per grams of original salt used.

In FIGURE 2 there is illustrated a second mode of operation of a plating bath in accordance with the present invention. In certain respects, this system, while slightly more complicated, possesses advantages which are not found in the system shown schematically in FIGURE 1. The system of FIGURE 2 utilizes an oxygenation bath 11 in conjuntcion with an actual plating bath 21. Pump means 22 and return path means 23 serve to circulate the plating solution between the two baths. Of course, the minimum rate at which one will need to circulate the solution will be dependent upon the rate at which plating is occurring and the temperatures at which the baths are operated. Optimum rates can be determined readily for any particular system used. For most plating systems, the scheme of FIGURE 1 will suffice; however, I have found that in the plating of magnetic elements as will be described hereinbelow, uniformity is much more readily obtained through the system such as shown in FIGURE 2 due to the lack of agitation and solution nonconformity which tends to occur in the bath of FIG- URE 1.

As has already been noted, the use of oxygen in a plating bath in accordance with the above description permits increased hypophosphite ion concentration. Referring to FIGURE 3, there is shown a plot of plating rate per unit time in arbitrary units versus increasing hypophosphite ion concentration in a mixed cobalt-nickel bath as described above. As can be seen, the plating rate increased on a linear scale with increasing concentration of hypophosphite ion. However, previous investigators have found that when the concentration limit of hypophosphite ion reaches approximately 30 gm./1., these baths spontaneously decompose within a very few minutes. When oxygen is bubbled through a bath at the rate of to bubbles per second per liter of solution, it has been found that hypophosphite ion can be increased to as high as 96 gm./ 1. Of course, as can be seen from the chart of FIGURE 3, the plating rate is no longer increased at concentrations above approximately 48 gm./l. However, the permissible increase concentration of hypophosphite ion above 48 gm./l. can still prove to be useful in view of the fact that this ion will not have to be replenished in the bath as frequently as has been required heretofore.

While the mixed cobalt-nickel plating bath shows peak plating rate occurring at about 48 gm./l., hypophosphite, a plain electroless nickel bath of the type previously described, showed the following rate.

H PO Weight of deposit, mg. gm./l. gm./l. 5 gm./l. 5.75

Plating was done on nickel metal strips having a surface of 0.25 in. for a period of 30 minutes with oxygen passing through at 15 to 20 bubbles per second per liter of solution.

As is well known in the art, certain metals are catalytic to the electroless nickel, nickel-cobalt and cobalt type systems. Likewise, methods are known for making noncatalytic surfaces catalytic. As these matters are well known and as they are not part of the present invention, they will not be discussed further herein.

In addition to the advantage of producing thicker plating per unit time, my invention provides still further advantages. The magnetic switching properties of electroless deposited nickel-cobalt is improved with increasing plating rate. I have found that properties heretofore unobtainable with conventional electroless plating tech niques may be realized through the use of the present invention. For example, in a plating bath of the cobaltnickel type codeposit described above and with oxygen passing therethrough at a rate of 10 to 15 bubbles per minute per liter of solution, deposits were obtained which possessed coercivity of 1:5 to 2.5 oersteds. Due to the low coercivity, the film possesses highly desirable properties for magnetic memory and logic units. In order to obtain useful magnetic properties, the bath must additionally contain about 0.33 mg./l. thiourea. Also in order to obtain an oriented deposit, an AC. or DC. field in excess of the H the coercivity of the deposit, is applied across the surface in the direction of desired orientation. In FIG- URE 2 there is schematically illustrated a body to be plated 24 interposed between the poles of a strong magnet 25 to provide the orienting magnetic field.

As an example of the improved megnetic results obtainable through the use of my invention, the following tests were run. Samples of nonmagnetic base materials were plated in accordance with the composite nickel-cobalt bath described above and in a bath identical to the above with the exception that the hypophosphite ion concentration was 50 gm./l. rather than the 83.3 gm./l. indicated in the above example. The properties of the deposit obtained through the two different baths having varying concentration of hypophosphite ion were as follows:

Rate of Deposition, gm. /sq. in

Hypophosphite Ion Concentration S u w( u) Where Tfiswitchin g time H applied field H threshold field S switching coefiicient As can be seen, the switching coeflicient S decreases with increasing plating rate. It is apparent from the above equation that switching time T is directly proportional to S The desirability of the rapid switching time at low drive is, of course, obvious to those using magnetic memory and logic units.

As has already been noted, thiourea and. other related materials have been known to possess utility in class II type stabilization of electroless plating baths. I have found that these materials may be used in conjunction with the present invention to produce both added stability and also to serve in a further useful way. For example, in a bath of the mixed nickel-cobalt type described above using oxygen as a class I stabilizer and a small quantity of thiourea as a class II stabilizer (from -500 lg/l.) the ratio of nickel to cobalt may be controlled by varying the thiourea content. By variation of the ratio of nickel to cobalt, the coercivity of the deposits may be tailored to the desired figures.

The following experiments will serve to illustrate the effect of oxygen and thiourea as controls for ultimate deposit composition in a mixed nickel-cobalt bath of the type previously described.

If one prepares a plating bath of cobalt-nickel and hypophosphite ion as illustrated above and inserts two electr0desone of nickel which is catalytic and one of lead which is noncatalytic-and measures the potential therebet'ween, the following is seen. (Of course, the bath is at constant plating temperature of 200 F.) Plating immediately commences upon the nickel electrode and an of 174 mv. is observed between the electrodes. If oxygen is now coupled into the bath, a fairly rapid change in is observed to about 194 mv. This lower will be maintained as long as oxygen is bubbled through the solution. If the bubbling of oxygen is stopped, the will gradually return to 174 mv. over a period of about 5 minutes.

If nitrogen is bubbled through a solution as in the previously described experiment prior to the bubbling of oxygen, the E.M.'F. Will remain essentially unchanged and may even rise slightly to about mv.

If a bath which has had oxygen bubbling therethrough and which has a potential of about 194 mv. between the electrodes now has nitrogen bubbled through, it will be found that the potential will decay back to the 174 mv. within about 30 seconds. This experiment clearly shows that the oxygen has an effect more than mere agitation.

If thiourea is added to a bath prior to the bubbling of oxygen therethrough, the potential of the bath will rise above l74 mv.

The significance of the above experiments is now believed obvious. By regulation of the quantities of oxygen and of thiourea, one may obtain results in accordance with the following observations.

(1) Cobalt content of the deposit decreases when oxygen is passed through the plating solution.

(2) Cobalt concentration of the deposit increases when thiourea is added to the plating solution.

Thus by means of combining class I and class II stabilizers, one may vary the ratio of cobalt to nickel in the deposit and thereby tailor the coercivity, within limits, to desired values.

Having thus described my invention, I claim:

1. The process of chemically plating a body of which at least surface portions are catalytic with at least one metal of the class consisting of nickel and cobalt, which comprises providing a hot aqueous plating bath including cations of at least one metal of the group consisting of cobalt and nickel and including anions of hypophosphite, maintaining the oxygen content of said bath at near saturation level, contacting said body with said hot plating bath, and allowing said body to remain in contact with said bath until the desired thickness of metal is deposited thereon.

2. The process of chemically plating a body of which at least surface portions are catalytic with nickel which comprises providing a hot plating bath including nickel ions and hypophosphite ions, maintaining the oxygen content of said bath at near saturation level, contacting said body with said hot plating bath, and allow-ing said body to remain in contact with said bath until the desired thickness of metal is deposited thereon.

3. The process of claim 2 wherein the hypophosphite ion concentration is from about 17 gm./l. up to about 96 gm./l. 4. The process of chemically plating a body of which at least surface portions are catalytic with a codeposit of nickel and cobalt which comprises providing a hot aqueous bath including cations of nickel and cobalt and hypophosphite ion, maintaining the oxygen content of said bath at near saturation level, contacting said body with said hot bath and allowing said body to remain in contact with said bath until the desired thickness of metal is deposited thereon.

5. The process of chemically plating a body of which at least surface portions are catalytic with at least one metal of the class consisting of nickel and cobalt, which comprises providing a hot aqueous bath including cations of at least one metal of the group consisting of cobalt and nickel and including anions of hypophosphite in concentration from about 17 up to 96 grams per liter, maintaining the oxygen content of said bath at near saturation level, contacting said body with said hot plating bath, and allowing said body to remain in contact with said bath until the desired thickness of metal is deposited thereon.

6. The process of claim 5 wherein the cations are both nickel and cobalt and the concentration of hypophosphite is about 4 8 gm./l.

7. The process of chemically plating a non-magnetic 8 body of which at least surface portions are catalytic with a nickel-cobalt alloy possessing magnetic properties useful for memory units which comprises providing a hot aqueous plating bath including nickel ions, cobalt ions, hypophosphite ions, and from -500 ,ug./1. of thiourea, maintaining the oxygen content of said bath at near saturation level, placing said body in said plating bath, applying a magnetic field across the surface of said body greater than H the coercivity of the deposit, and allowing said body to remain in said bath until the desired thickness of metal is deposited thereon.

8. The process of chemically plating a non-magnetic body of which at least,the surface portions are catalytic with a metallic coating having improved magnetic switching properties which comprises preparing a solution of about'0.25 M cobalt ion, about 0.23 M nickel ion, 06 M ammonium ion, about 0.78 M hypophosphite ion, 0.34 M citrate ion, 0.33 mg./l. thiourea and a pH of about 7.5, maintaining said solution essentially saturated with oxygen, heating said bath to about 200 F., immersing said body in said =bath, applying a magnetic field in excess of 2.5 oersteds across the surface of said body, and allowing said body to remain in said bath until the desired thickness of metal is deposited thereon.

References Cited by the Examiner UNITED STATES PATENTS 2,819,188 1/1958 Metheny et a1. 2,929,742 3/1960 Minjer et al. 2,938,805 5/1960 Agens 117-130 X MURRAY KATZ, Primary Examiner.

RALPH S. KENDALL, Examiner. 

1. THE PROCESS OF CHEMICALLY PLATING A BODY OF WHICH AT LEAST SURFACE PORTIONS ARE CATALYTIC WITH AT LAST ONE METAL OF THE CLASS CONSISTING OF NICKEL AND COBALT, WHICH COMPRISES PROVIDING A HOT AQUEOUS PLATING BATH INCLUDING CATIONS OF AT LEAST ONE METAL OF THE GROUP CONSISTING OF COBALT AND KICKEL AND INCLUDING ANIONS OF HYPOPHOSPHITE, MAINTAINING THE OXYGEN CONTENT OF SAID BATH AT NEAR SATURATION LEVEL, CONTACTING SAID BODY WITH SAID HOT PLATING BATH, AND ALLOWING SAID BODY TO REMAIN IN CONTACT WITH SAID BATH UNTIL THE DESIRED THICKNESS OF METAL IS DEPOSITED THEREON. 